1. Field of the Invention
The present invention generally relates to a system for improving the measurement accuracy of a variable resistance transducer such as a strain gauge. The system generates two voltages. The first voltage is a function of the resistance change measured by the transducer, and the second voltage is representative of the ambient temperature of the transducer. These generated voltages are achieved with very few lead wires and without the need for a second transducer. In addition, the measurements are uncontaminated by lead wire resistance effects.
2. Description of the Related Art
Several approaches are currently available for measuring resistance and temperature parameters in variable resistance transducers. These approaches are generally based on using separate transducers and wiring from the measurement system to the article to be measured. For example, the classic Wheatstone bridge electrical circuit is typically used for measuring small variations in resistance, for example, to measure resistance changes when a strain gage is used as a transducer. The Wheatstone bridge may also include a second resistor to compensate for the temperature effects experienced by the main transducer resistance. An additional method of removing measurement error encountered when using the Wheatstone bridge includes measuring the transducer temperature using a separate thermocouple or resistance temperature device (RTD).
In addition, constant current excitation has been used in an attempt to achieve a linear output and avoid parasitic resistance problems when measuring resistance and temperature using, for example, a transducer. Constant current excitation can also achieve double the output voltage for a given power dissipation in the resistance transducer when compared to voltage divider circuits such as the Wheatstone bridge. "Constant Current Loop Signal Conditioning", the subject of a pending patent application, Ser. No. 08,018,128, filed Feb. 16, 1993, is an innovative form of constant current signal conditioning that uses a form of output voltage processing to completely eliminate lead wire resistance effect. Constant current loop signal conditioning is also discussed in NASA Technical Memorandum 104260 by Karl F. Anderson, "The Constant Current Loop: A New Paradigm for Resistance Signal Conditioning." Both the Patent application Ser. No. 08/018,128 and the NASA Technical Memorandum 104260 are hereby incorporated by reference.
One of the problems encountered using the Wheatstone bridge is that the output is generally nonlinear with respect to the resistance change. Within the Wheatstone bridge are found various electrical connections and lead wires that attach the resistance to be measured to the rest of the Wheatstone bridge circuit. The Wheatstone bridge circuit may also contain additional components, such as slip rings for connecting rotating machinery and fuses for electrical fault protection. These various parasitic (i.e., present but undesirable) resistances will themselves vary due to the thermal, mechanical, chemical and other conditions of the environment. This variation in resistance may develop errors in measurement which are not always practical or easy to correct.
An example of a circuit known in the art which attempts to reduce the effects of parasitic resistance is a circuit which connects three wires to a remote variable resistor disposed in the environment. This three-wire circuit attempts to electrically subtract the parasitic resistance variations in each of the current carrying leads connected to the variable resistor. The parasitic resistances are effectively canceled at the output of the Wheatstone bridge by connecting the leads of adjacent arms of the Wheatstone bridge. This approach is effective in moderate temperature environments. The measurement system becomes less sensitive due to the increase in circuit resistance caused by the lead wires. In addition, since the wires and associated components are not identical, in severe temperature environments, the parasitic resistances vary greatly. This results in an unreliable output and inaccurate measurement. Thus, the prior art has been unable to measure resistance changes and temperatures which are unaffected by parasitic resistances or voltages.
It is, therefore, desirable to reliably measure resistance changes and temperature unaffected by external conditions such as parasitic lead wire resistances and thermoelectric effects which cause measurement errors. In addition, it is also desirable to minimize the number of conductors required for measuring resistance changes and temperature. Further, it is desirable to measure temperature in the presence of parasitic resistances resulting from environmental or external conditions.